The present invention is related to a method for handling parallel switching planes in a packet switch and also a packet switch including such handling.
In order that a switch in a network for the transfer of data packets, for instance an ATM network, will be able to satisfy the requirement of a high reliability, some form of redundancy is required, i.e. that some parts of the hardware of the switch are made as double or multiple units, which perform exactly the same functions and operate in parallel to each other. A common way of achieving this redundancy is by means of parallel and independent switching planes.
The transfer of data packets in a packet switch may be incurred with errors, so that for instance individual data packets disappear, will be wrongly addressed or that bit errors appear in the data packet. These errors can then be detected, when the data packet is to be forwarded from the packet switch. If several switching planes are provided, the task thus is to grasp and forward, at the output port of the packet switch, only the data packets transferred without errors and also monitor that the data packets are forwarded in a correct sequential order. In packet networks and particularly ATM-networks it is, to some extent, in addition permitted that data packets are lost or discarded. For instance, the buffers necessary for the handling of the statistical multiplexing in a packet switch can be filled, whereafter data packets arriving to the buffer may be lost. Having a redundancy in the switch, for instance with parallel switching planes, this means that the same data packet in the different planes is not available at the same time at the output port of the switch, where the redundancy is terminated, that is in the connection point of the parallel switching planes. Some procedure is thus needed at the output side of such a packet switch for achieving a correct stream of forwarded packets.
The European patent application 0 381 334 discloses the handling of asynchronous, redundant digital data messages. Several parallel and equivalent communication channels are each one connected to an own buffer. The different data messages which have been received in an arbitrary order, are sorted and a majority selection is made in order to determine those channels which have correct data messages.
In the European patent application EP-A1 0 453 607 a packet switch is disclosed having redundant switching planes. For each output port from the switch there is an evaluation unit, which receives data packets from the different switching planes and in the common way selects data packets having no errors but also selects preferably or in the first place data packets from that switching plane on which the data packets are communicated most slowly. At the termination of the redundant planes it is tested if a data packet transferred on the slowest plane is the next one to be forwarded from the switch in a correct sequential order. If it is the case, the packet is forwarded, and otherwise it is cancelled by the packet next transferred being written on top of the first data packet.